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Conversion of Fuel-N into HCN and NH3 During the Pyrolysis and Gasification in Steam: A Comparative Study of Coal and Biomass† Fu-Jun Tian,‡ Jianglong Yu,‡ Lachlan J. McKenzie,‡ Jun-ichiro Hayashi,§ and Chun-Zhu Li*,‡ Department of Chemical Engineering, PO Box 36, Monash UniVersity, VIC 3800, Australia, and Centre for AdVanced Research of Energy ConVersion Materials, Hokkaido UniVersity, N13-W8, Kita-ku, Sapporo 060-8628, Japan ReceiVed August 20, 2006. ReVised Manuscript ReceiVed December 8, 2006
Conversion of fuel-N into HCN and NH3 during the pyrolysis and gasification of coal and biomass in steam were compared using fluidized-bed/fixed-bed and two-stage fluidized-bed/tubular reactors. During the pyrolysis and gasification of coal and biomass in steam, the thermal cracking of volatile-N was the main route for the formation of HCN while a small amount of HCN was formed from the breakdown of relatively unstable N-containing structures in char. Our results indicate that once the fuel-N in both biomass and coal is condensed/ polymerized into the solid-phase char-N during the gasification in steam, the main nitrogen-containing gaseous product from char-N would be NH3. However, the thermal-cracking/reforming of volatile-N constitutes an additional important route of NH3 formation during the gasification of biomass (e.g., cane trash) in steam while this route is negligible for the gasification of coal. The selectivity of char-N toward HCN and NH3 is largely controlled by char-N stability and/or the availability of H and/or other radicals during the gasification of coal and biomass.
Introduction Much attention has been focused on NOx reduction during the development of gasification-based technologies for power generation using coal and biomass in order to meet the increasingly stringent future environmental standards. During the gasification of coal and biomass, the two major precursors of NOx and N2O are HCN and NH3.1-9 The gasification-based technologies may provide the possibility to remove NOx precursors (e.g., HCN and NH3) before combustion in gas turbines. However, the removal of these gaseous species could greatly increase the capital and maintenance costs of power generation. Minimizing the formation of NOx species and their precursors inside gasifiers during gasification will be an optimum option. Understanding the conversion of fuel-N in coal and biomass during pyrolysis and gasification is essential for the choice of optimum reaction conditions in future gasifiers. Nitrogen functionalities in biomass are different from those in coal. In biomass, nitrogen mainly exists as proteins (and † Presented at the 2006 Sino-Australia Symposium on Advanced Coal Utilization Technology, July 12-14, 2006, Wuhan, China. * Corresponding author. E-mail:
[email protected]. ‡Monash University. § Hokkaido University. (1) Aho, M. J.; Hamalainen, J. P.; Tummavuori, J. L. Fuel 1993, 72, 837. (2) Aho, M. J.; Paakkinen, K. M.; Pirkonen, P. M.; Kilpinen, P.; Hupa, M.; Combust. Flame 1995, 102, 387. (3) Hamalainen, J.; Aho, M. Fuel 1996, 75, 1377. (4) Hamalainen, J. P.; Aho, M. J.; Tummavuori, J. L. Fuel 1994, 73, 1894. (5) Li, C.-Z. Conversion of Coal-N and Coal-S during Pyrolysis, Gasification and Combustion. In AdVances in the Science of Victorian Brown Coal; : Li, C.-Z., Ed.; Elsevier: New York, 2004; Chapter 6. (6) Leppalahti, J. Bioresour. Technol. 1993, 46, 65. (7) Leppalahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1. (8) Tian, F.-J.; Li, B.; Chen, Y.; Li, C.-Z. Fuel 2002, 81, 2203. (9) Li, C.-Z.; Tan, L. Fuel 2000, 79, 1899.
amino acids)7 together with some other forms such as DNA, RNA, alkaloids, porphyrin, and chlorophyll. On the contrary, nitrogen mainly exists in heteroaromatic ring systems in coal: pyrrolic-nitrogen and pyridinic-nitrogen are the two major forms of nitrogen in coal.5,10 The differences in the fuel-N functionalities may affect the conversion of fuel-N toward HCN and NH3 during the pyrolysis and gasification of coal and biomass. For example, the thermal cracking of volatiles is seen to be one of the main routes of NH3 formation during the pyrolysis of biomass11 but not during that of coal.9,12,13 In the case of HCN formation, the results from the pyrolysis of coal9,12,13 and the pyrolysis of biomass8,11 all indicate that HCN was mainly formed from the thermal cracking of volatiles during pyrolysis. However, there is a lack of knowledge about the effects of nitrogen functionality on the conversion of fuel-N during gasification. During pyrolysis and gasification, the distribution of fuel-N into volatile-N and char-N is significantly dependent on fuel rank. While a large fraction of biomass-N would become volatile-N the pyrolysis of high rank coal would retain the majority of coal-N as char-N. The differences in the relative proportions of volatile-N and char-N from biomass and coal could be a critical factor in controlling the conversion of fuel-N into HCN and NH3. Under gasification conditions, the presence of gasifying agents (e.g., steam) could reform volatile-N and gasify char-N, leading to the formation of HCN and NH3. It is still unclear how the relative partition of fuel-N into volatile-N (10) Davidson, R. M. Nitrogen in coal; IEAPER/08, IEA Coal Research: London, UK, 1994. (11) Tian, F.-J.; Yu, J.-l.; McKenzie, L.; Hayashi, J.-i.; Chiba, T.; Li, C.-Z. Fuel 2005, 84, 371. (12) Tan, L.; Li, C.-Z. Fuel 2000, 79, 1883. (13) Tan, L.; Li, C.-Z. Fuel 2000, 79, 1891.
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Table 1. Properties of Biomass and Coal Samples Used ultimate analysisd (wt %) coal samples
particle size, µm
sewage sludge cane trash Loy Yang coal Drayton coal Shenmu coal Dongshan coal
106 - 150 125 - 212 106 - 150 106 - 150 106 - 150 106 - 150
a
moisturea
wt %
5.70 6.00 10.00 3.08 5.24 1.00
ashb wt
%
50.9 7.6 1.0 5.5 5.1 10.7
VMc,d wt 86.7 86.5 51.5 34.4 32.9 13.2
%
FCd,e wt 13.3 13.5 48.5 65.6 67.1 86.8
%
C
H
N
S
Of
52.5 49.5 68.5 82.2 82.3 90.0
8.35 6.10 4.80 5.50 4.53 3.81
8.07 0.31 0.55 1.76 1.03 1.29
2.08 0.08 0.32 0.97 0.19 2.81
29.0 44.0 25.8 9.6 12.0 2.1
Raw coal. b Dry basis. c VMsvolatile matter yield. d Dry and ash-free basis. e FCsfixed carbon. f By difference.
and char-N, influenced by fuel rank, would impact the selectivities of HCN and NH3 during the gasification of coal and biomass. This study aims to understand the effects of fuel rank on the conversion of fuel-N during gasification in steam. A set of fuels ranging from cane trash, and sewage sludge, to Loy Yang brown coal and higher rank coals have been gasified in steam and the formation of HCN and NH3 will be reported here. Experimental Details Samples. Samples used in this study include a cane trash, a sewage sludge, a brown coal (Loy Yang coal), and three bituminous coals (Drayton, Shenmu, and Dongshan). Their properties are given in Table 1. The cane trash and the sewage sludge were chosen and treated as the youngest “coals”. Pyrolysis and Gasification. A one-stage fluidized-bed/fixedbed reactor was used to carry out the pyrolysis and gasification of coal/biomass with steam at fast heating rates. The detailed configuration of this reactor has been given elsewhere.14 Briefly, there were two quartz frits placed in the reactor. The bottom one acted as the gas distributor and supported about 170 g of zircon sand (used as the fluidized-bed material). The top frit was placed in the freeboard to prevent the char particles from being elutriated out of the reactor. As a result, it produced a thin char bed where volatiles could interact with the char bed when passing through the frit. Therefore, this reactor has combined features of a fluidized-bed reactor (e.g., fast heating rates) and of a fixed-bed reactor (e.g., a thin char bed). In a pyrolysis run, two streams of ultra high purity argon (>99.999%) were used as a feeding gas and a fluidizing gas. First, the reactor was heated to the required temperature using an electrical furnace. Second, biomass or coal particles were fed into the reactor through a water-cooled probe that prevented the biomass or coal particles from being pyrolyzed before meeting the fluidized bed. When the particles entered the hot fluidized sand bed, they were heated at fast heating rates (>103 K min-1). In a gasification run, two streams of argon gas were also used as a feeding gas and a fluidizing gas. However, when the reactor was heated to the required temperature, an HPLC pump was used to feed deionized water at a constant flow rate into the reactor (just beneath the bottom frit) to generate steam. The steam formed underneath the bottom frit was immediately mixed with the fluidizing gas and swept into the fluidized bed. Thermal Cracking and Reforming of Volatile-N. Upon pyrolysis at 600 °C, the majority of volatiles were released during the pyrolysis of coal and biomass in a fluidized-bed reactor.14-16 The thermal cracking and reforming of volatile-N in steam was carried out using a two-stage fluidized-bed/tubular reactor described elsewhere.14 The first stage was a fluidized-bed reactor, identical to the above one-stage fluidized-bed/fixed-bed reactor, to generate nascent volatiles from pyrolysis at 600 °C at fast heating rates. The volatiles were then thermally cracked or reformed in steam in (14) Xie, Z.; Feng, J.; Zhao, W.; Xie, K.-C.; Pratt, K.; Li, C.-Z. Fuel 2001, 80, 2131. (15) Li, C.-Z.; Nelson, P. F. Energy Fuels 1996, 10, 1083. (16) Keown, D. M.; George, F.; Hayashi, J.-i.; Li, C.-Z. Bioresour. Technol. 2005, 96, 1570.
the second-stage tubular reactor held at a preset temperature between 600 and 1000 °C. Steam was introduced directly into the tubular reactor via a supplementary gas inlet14 so that the reforming of volatiles with steam (a gasifying agent) took place without the steam being in contact with the corresponding char held in the first stage. In all runs, the nominal feeding rate of coal/biomass was 30 mg min-1. In all gasification/reforming cases, the steam concentration was fixed at 15% (by volume). Quantification of HCN and NH3. NH3 in the product gas bubbled through and was absorbed in a 0.02 M methylsulfonic acid (MSA) solution and HCN was absorbed in a 0.1 M NaOH solution. HCN or NH3 were collected into reaction-time-resolved fractions by changing the absorption bubblers connected to the exit of the reactors. NH4+ and CN- absorbed in solutions in separate experiments were quantified using Dionex 500 ion chromatographs with separate analytical methods following the procedures described elsewhere.12
Results and Discussion Formation of HCN and NH3: Similarities and Dissimilarities between Cane Trash and Loy Yang (LY) Brown. Figure 1 shows the reaction-time-resolved accumulated yields of NH3 during the pyrolysis and gasification of cane trash and LY brown coal in steam. The first points in these figures refer to the yields from the “feeding” periods, during which coal/ biomass particles were continuously fed into the reactor. The other points refer to the yields from the “not-feeding” periods, in which the feeding of coal/biomass had stopped and the char inside the reactor continued to be pyrolyzed or gasified. During the pyrolysis of cane trash, the majority of NH3 was formed in the “feeding” periods (Figure 1a-c). There are two pathways to form NH3: the hydrogenation of char-N by H radicals or the thermal cracking of volatile-N in the gas phase. Due to the presence of the frit in the freeboard of the one-stage fluidized-bed/fixed-bed reactor,14 there would be strong interactions between the volatiles and char. The volatiles generated within the fluidized bed contain H-rich structures and would readily be thermally cracked to generate radicals. As the radicalrich volatiles passed through the frit, the radicals (particularly H-radicals) would react with the nascent char held underneath the frit to form NH3. In addition, the cane trash particles fed into the reactor were not completely dried and contained 6% of moisture. Furthermore, the high (∼44%) oxygen content of the cane trash (see Table 1) means that a significant amount of H2O and other O-containing species (e.g., COx) would be produced as pyrolysis products. Additional H radicals can therefore be generated for the “self-gasification” of cane trash by H2O as moisture in the trash or pyrolytic H2O. During pyrolysis under the current conditions, the self-gasification of char would convert char-N into NH3. Clearly, this source of H can only be generated during the feeding period when the two sources of water (and other reactive species) are available/ generated. The additional yields of NH3 in the presence of externally fed steam (Figure 1a-c) are significant evidence to
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Figure 1. Accumulated NH3 yields with reaction time during the pyrolysis and gasification of cane trash and Loy Yang brown coal with steam: (O) gasification; (b) pyrolysis.
the formation of NH3 through the self-gasification of char with inherent moisture and pyrolytic water during pyrolysis. In addition to the factors described above, during pyrolysis, the cane trash released up to 80% of its mass as volatiles16 and some volatiles could be cracked or self-reformed by H2O to produce NH3 within the fluidized bed and in the freeboard. This possibility of NH3 formation was further tested by extending the secondary reactions of volatiles in the gas-phase using the two-stage reactor. As is shown in Figure 2, compared with the NH3 yields of less than 5% coal-N produced during the pyrolysis of the same cane trash at 600 °C, the 22-27% NH3 yields observed with the two-stage reactor must have come from the reactions of volatiles in the gas phase. The data in Figure 2a also suggested that very little, if any, additional NH3 is likely to be formed upon the introduction of steam. However, the data in Figure 2a indicated that additional NH3 was not formed from the thermal cracking/reforming of volatiles during the pyrolysis and gasification of LY brown coal. This dissimilarity is attributed to the different nitrogen functionalities in cane trash and LY brown coal: the proteins (and amino acids) are rich in cane trash but these are negligible in the LY brown coal. Unlike the pyrrolic-N and pyridinic-N structures in coal-volatiles, the proteins (and amino acids) or their derivatives in biomassvolatiles were thermally cracked into HNCO which could be hydrolyzed into NH3 in the absorption systems used in this study. The drop in the yields of NH3 (Figure 2a, brown coal) is possibly due to the interaction of NH3 and the quartz wall, resulting in the loss of NH3.17 With the introduction of H2O, the NH3 yields from both biomass-N and coal-N were drastically increased. In the case (17) Li, C.-Z.; Nelson, P. F. Fuel 1996, 75, 525.
of cane trash, introducing steam into the gas phase did not affect the formation of NH3 from the secondary reactions (thermal cracking or reforming) of volatiles. The hydrogenation of char-N by H radicals is one of the major routes for NH3 formation from biomass-N. The increases in the accumulated yields of NH3 in the not-feeding period during the gasification of cane trash (Figure 1) indicated that the increased NH3 yields by the externally added steam must have been (partly) from the hydrogenation of char-N by H radicals. The introduction of H2O may also increase the concentration of other radicals (e.g., OH or even O) on the char surface. It appears that these radicals are mainly involved in the consumption of the char matrix. They may also speed up the activation and breakdown of N-containing ring structures. However, the hydrogenation of char-N by H radicals appears to be the dominating factor influencing the ultimate formation of NH3. However, the gasification of char in the not-feeding periods did not form HCN (Figure 3). It is possible that the formation of HCN from the gasification of char was so fast that it was complete within the feeding periods. The decreased HCN yields during the gasification of cane trash (compared with the yields of HCN during the pyrolysis of cane trash) (Figure 3) might be a result of the changes in the selectivity of char-N toward HCN and NH3, owing to the increased availability of H radicals. The additional H radicals could be generated from the enhanced reforming of volatiles and the gasification of char during the gasification of cane trash in steam. A closer examination of NH3 (Figure 1d-f) and HCN (Figure 4) formed from the gasification of LY brown coal further confirmed the changes in the selectivity of char-N into HCN and NH3 due to the increased availability of H radicals due to
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Figure 4. Accumulated yields of HCN with reaction time during the pyrolysis and gasification of LY brown coal with steam in the onestage fluidized-bed/fixed-bed reactor at 700 and 900 °C.
Figure 2. Yields of HCN and NH3 from the thermal cracking/reforming of volatiles from LY brown coal and cane trash as functions of temperature in the two-stage fluidized-bed/fixed-bed reactor. The bottom stage was set at 600 °C.
Figure 5. Formation of NH3 from the gasification of different fuels with steam in the one-stage fluidized-bed/fixed-bed reactor at 700 °C.
Figure 3. Accumulated yields of HCN with reaction time during the pyrolysis and gasification of cane trash with steam in the one-stage fluidized-bed/fixed-bed reactor at 600 and 900 °C.
the introduction of steam. During the gasification of LY brown coal in steam, considerable amounts of NH3 were formed in the not-feeding periods (Figure 1). Figure 4 shows that the yields of HCN also increased upon the introduction of steam. Comparison of the data in Figures 1 and 4 indicates that the increased availability of H radicals (mainly due to the interaction of char and steam) favored the formation of NH3 significantly over that of HCN, and the difference between NH3 and HCN mainly occurred during the char gasification (i.e., in the notfeeding period). It is also likely that some relatively unstable N-containing structures (e.g., small heteroaromatic ring structures) could have been present in char or formed as the macromolecular structures of char broke down. These unstable structures could be easily thermally cracked down into HCN or its light precursors. The low yield of HCN at 700 °C (Figure 4) is due to the small amount of these unstable structures in char during gasification in steam. Increasing the reaction
temperature increases the possibility of forming these unstable structures in char. The increase in the yield of HCN at 900 °C (Figure 4) is higher than that at 700 °C. Discussion of Fuel-N Conversion during the Pyrolysis and Gasification of Coal and Biomass with Steam. Figure 5 shows the effects of fuel rank on the formation of NH3 during the gasification in steam with a long holding time at 700 °C for the complete gasification of char. During the periods of feeding and the early not-feeding stages, the gasification of biomass (cane trash and sewage sludge) produced higher NH3 than that of coal. As was aforementioned, during the gasification of cane trash, the routes for NH3 formation are the thermal cracking/ reforming of volatiles and the hydrogenation of char-N. During the feeding periods, it is most likely that the high NH3 yield from the gasification of cane trash and sewage sludge is mainly due to their high volatile yields, though the contribution of the gasification of char-N to the NH3 formation cannot be excluded. In the not-feeding periods, due to the relatively high reactivity of biomass-char (compared to coal-char), the formation of NH3 went to completion within a holding time of 400 min (after the experiments, all char was observed to be gasified). With a holding time of around 400 min, the coal-char gasification did not go to completion. The gasification of coalchar took significantly longer than the gasification of biomasschar. With extending the holding time, the accumulated yield of NH3 from the gasification of coals exceeded that of biomass. The accumulated yield of NH3 was nearly 70% coal-N. Figure 6 shows the effects of fuel rank on the formation of HCN during the gasification with steam at 700 °C with a long
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Dongshan coal) the amount of relatively unstable structures is small so that the yield of HCN during the gasification of Dongshan-coal-char is only around 2% (Figure 6). Nevertheless, even for the coal of the highest rank in this study, a small portion of char-N was still converted into HCN. Conclusion
Figure 6. Formation of HCN from the gasification of different fuels with steam in the one-stage fluidized-bed/fixed-bed reactor at 700 °C.
holding time. There are two possible routes for the formation of HCN from fuel-N during the gasification in steam. The first route is HCN formed through the thermal cracking of volatiles. The second route of HCN formation is from the gasification of char-N. However, compared with the yield of NH3 (Figure 1), the accumulated yield of HCN is low. The low concentration of the relatively unstable structures in char and the high availability of H-radicals most likely combined to result in the high NH3 yield of coals (Figure 5) and the low HCN yield of coals (Figure 6). It seems that in the higher rank coal (e.g.,
During the gasification of coals, the availability of H-radicals is the critical key in controlling the conversion of char-N into NH3 and HCN. The thermal cracking of volatile-N is the main HCN formation route, but small amounts of HCN are formed from char-N. During the gasification in steam, the high NH3 yields are a result of the increased availability of H-radicals, mainly from the interaction of char and H2O. During the gasification in steam, once the fuel-N is retained in the form of char-N, most of the fuel-N would tend to be converted into NH3. Furthermore, the more stable the char-N structures, the higher the NH3 yield. Acknowledgment. The authors gratefully acknowledge the financial support of this study by the New Energy and Industrial Technology Development Organisation (NEDO) in Japan. This project was partly supported proudly by the International Science Linkages established under the Australian Government’s innovation statement, Backing Australia’s Ability. Helpful discussion with Prof. L. Chang of Taiyuan University of Technology is gratefully acknowledged. F-J Tian gratefully acknowledges the MIPRS and MGS scholarships from Monash University. EF060415R
